The high-vacuum deposition of thin films, such as Cu(InGa)Se2, by thermal evaporation onto horizontally-oriented substrates which are spatially situated above the evaporation source (herein referred to as “vertical evaporation”) is well known, and may be useful for forming absorber layers for photovoltaic devices. Generally speaking, a vertically-evaporating thermal evaporation source comprises a substantially closed vessel containing an evaporant, typically in liquid but possibly in solid form, with at least one effusion nozzle tunneling through the upper surface of the vessel through which the elemental vapor effuses. The relative simplicity of the effusion source design is one of the significant advantages of vertical evaporation.
However, a problem with vertical (i.e., upward) evaporation is that the substrate, in particular a rigid substrate, may only be supported at its edges to avoid either shadowing the substrate surface from deposition, or marring the substrate surface by physical contacting. The restriction of supporting the substrate at its edges can for some substrates limit the substrate temperature during deposition. One particular example is glass and more particularly soda-lime glass, where using an excessively high substrate temperature (such as in the vicinity of the softening point in the case of glass) can cause warpage or breakage of the substrate. This limiting of the substrate temperature may ultimately limit the desired properties of the deposited film, such as the photovoltaic conversion efficiency of Cu(InGa)Se2 absorber layers on soda-lime glass, as it is well known that the photovoltaic conversion efficiency of solar cells utilizing Cu(InGa)Se2 absorber layers typically increases monotonically with substrate temperature up to a temperature of approximately 550° C.
The most basic requirements of a thermal evaporation source are a volume comprising the elemental source material, and single or plural effusion nozzles to direct the elemental vapor, generated by the melt surface, from the source interior to the substrate. In the case of a vertically-evaporating source, the effusion nozzles will ideally be within close proximity to and axially oriented normal to the melt surface. In the simplest designs, the effusion nozzles will be aimed vertically and located directly above the melt surface, as illustrated in FIG. 1. FIG. 1 shows a prior art vertical evaporating source 30 with effusion nozzles 36 passing through heat shielding 22 and situated directly above and in close proximity to the surface of evaporant material 14. The substrate to be coated is indicated at 26. The device also comprises an evaporation chamber 18 and a containment box 12.
It is desirable to minimize the external surface area of the source in order to minimize the thermal load. Further, it is desirable to minimize the aspect ratio of the source (the ratio of the major dimension to the minor dimension, upon viewing the surface of the evaporant, so as to maximize temperature uniformity within the source. A non-uniform melt temperature results in variations in vapor pressure above the melt, causing variations in effusion rate through the nozzles, ultimately contributing to non-uniform film thicknesses on the substrate. Further hindering uniform deposition is the fact that the temperature profile of the source may be expected to change as depletion of the elemental source material occurs, thereby further reducing thermal conductance along the major axis and reducing deposition uniformity. A potential remedy to the problems exhibited by the configuration in FIG. 1 is the configuration described by Baron et al (U.S. Pat. No. 4,401,052), in which a separate low-aspect-ratio melt chamber is heated to generate a vapor of the evaporant from a substantially isothermal evaporant surface. This vapor is then directed into a manifold and out through multiple effusion nozzles to the substrate. A problem with this configuration is that in the case of evaporants which require very high temperatures for sufficient vapor generation, the large surface area of this configuration may result in an unacceptably high thermal loading. Furthermore, the actual physical fabrication of this design is challenging.